As well known, the industrial extraction of juice and puree from vegetable food—fruit and vegetables—executed by means of rotating machines, such as rough and fine extractors, is carried out squeezing a normally cut or chopped food through a perforated sheet or sieve. This sieve has cylindrical or conic shape and separates the parts of food to use, i.e. the product, from parts to be refused.
The energy necessary for the extraction is supplied by a driven impeller, or armature, which brings into rotation the chopped food within the sieve causing it to pass through the sieve by centrifugal force. The higher is the speed, the higher is the centrifugal force available for extraction of the puree through the holes of the sieve. The efficiency of the machine, for each type of starting food, is given by the ratio between the part of food that can pass outwards through the sieve, and the part that cannot pass through the sieve after having run across it for all its length, at the end of which it is disposed of.
Under equal conditions, the efficiency of extraction increases normally with the speed. However, after a certain time of operation of the machine, a thick layer, substantially consisting of puree that obstructs partially or totally the holes thereof, coats the outer surface of the sieve. This phenomenon, for most of food, is negligible in a first portion of the extraction surface of the sieve, and tends to increase gradually towards the final portion; in fact the juice is substantially a liquid and easily passes through the first portion of the sieve, whereas the puree, being thicker, encounters higher resistance to pass through the holes, and sticks on the external surface of the sieve owing to its viscosity. Therefore, the puree only partially leaves the outer surface of the sieve and obstructs the holes, causing a reduction of the efficiency of the machine.
Therefore, it is periodically necessary to remove the layer of thickened puree from the outer surface of the sieve for returning to a full extracting efficiency of the machine.
Solutions to this problem have been studied embodied in various devices, normally mechanical or hydraulic.
The former are scrapers or brushes having rotational or axial movement with respect to the sieve, so that they can raise the thick layer for detaching it and causing it to drop. The latter provide nozzles or sprayers emitting jets of washing liquid (mostly water) against the thickened layer, causing it to drop and clearing the outer surface of the sieve.
A feature common to the known systems is that they operate outside the sieve: this causes the cleaning forces, that are necessary for clearing the thick layer of deposited puree, to be directed from the outside towards the inside of the sieve, i.e. opposite to the puree flow, which is directed outwards.
This fact limits substantially the efficiency of these systems because the thickest parts of puree remain in the holes, especially in case of fine sieves, and hardens further within them.
Furthermore, in the systems using a washing liquid, strong jets of water must be used, with also a large quantity of water up to hundreds of liters, which adds to the juice or the puree causing an undesired dilution thereof.